If you've only got one, and assuming your partner doesn't have their own, then it would be nice to have two. Otherwise, when you get to the anchors and clip yourself to them, your belayer would then have to take you off belay and tie the device onto the rope so you can hull it up and use it. Now that I think about it I guess you're gonna have to hull the rope up anyway, but I prefer to just have a device with me.

Bizarre scenario. If you only have one belay device between the two of you - wtf are you doing attempting to rap?!? If you have to rap, rig a biner brake. (Oh, yeah - requires knowledge.) Planning a scenario where you need to haul a belay/rap device up to the climber is planning a scenario where there are just too many things that can go wrong. One device, it stays with the belayer who lowers the climber. No rap. Much better scenario - two belay/rap devices, one (belonging to and) with each climber. Not both with the belayer.